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The TJ Radio Relay System 

By J. GAMMIE and S. D. HATHAWAY 

(Manuscript received February 19, 1960) 

The TJ radio relay system is a broadband microwave facility that oper- 
ates in the 10,700- to ll,700-?nc common carrier frequency hand. It has 
been specifically designed for short-haul transmission of cither multichannel 
telephone or television circuits. Transmission performance and the over-all 
system description are presented, as well as some early field applications. 

Table of Contents 

I. Introduction 821 

II. Objectives 822 

2.1 Area of Applicfltion 822 

2.2 Development Objectives and Performance Characteristics 823 

2.3 Transmission Objectives 824 

III. Transmission Plan 825 

IV. Description of System 828 

4.1 General Description 828 

4.2 Diversity Switching Arrangements 845 

4.3 Nondiversity Applications 850 

4.4 Order-Wire, Alarm and Control System 850 

4.5 Antenna Systems 853 

4.6 Connecting Circuits 859 

4.7 Standby Power 861 

V. Equipment Features 862 

5.1 Transmitter-Receiver Bay 862 

5.2 Auxiliary Bay 865 

5.3 Interconnecting Arrangements 867 

VI. System Maintenance and Test Equipment 870 

VII. Applications of TJ Radio Systems 871 

7.1 Path Selection 871 

7.2 Typical TJ Installations 874 

VIII. Acknowledgment 877 

References 877 

I. INTRODUCTION 

During the past decade, microwave radio in the Bell System has 
had a phenomenal growth. In terms of route mileage and number of 
circuits, this growth has been primarily in the long-haul field in the 
4,000-mc common carrier band. The TD-2 system' - now criss-crosses 
the continent several times and pi-ovides facilities for telephone and 

821 



822 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1960 

television to almost every part of the continental United States.* In 
many areas it is already loaded to capacity and, hence, its use for 
other than backbone service is becoming increasingly restricted. In the 
next decade it is expected that there will be a large demand for short- 
haul microwave facihties along backbone routes and remote rural areas. 
These will have to be supplied by systems operating in common car- 
rier bands other than the 4,000-mc band. To allow for orderly growth of 
the Bell System radio plant, the 11,000-mc band has been selected 
for short-haul service needs where the maximum channel cross sec- 
tion might be only a few hundred telephone circuits. 

In the past, the telephone companies have used 4,000-mc TE equip- 
ment,^ secondary TD-2 arrangements or other currently available 6,000- 
mc common carrier equipment to fulfill their short-haul needs. Feasi- 
bility studies'" by the radio research group at Bell Telephone Laboratories 
and systems engineering studies made in cooperation with the telephone 
companies indicated the possibility of and need for a new economical 
short-haul system that would permit the dropping and adding of cir- 
cuits at each repeater or alternatively, be capable of transmitting mono- 
chrome or NTSC color television. Because of the potential interference 
problems with the 4,000-mc TD-2 system, and the 6,000-mc TH sys- 
tem^ now being installed in some sections of the country, the new TJ 
system has been developed for the 10,700-11, 700-mc common carrier 
band. 

II. OBJECTIVES 

2.1 Area of Application 

There arc a great number of uses for a flexible, economical short- 
haul radio system such as TJ. A partial list of these applications in- 
cludes : 

(a) relief on open-wire or cable routes now at full capacity; 

(b) added facilities along open-wire and secondary cable routes for 
improved reliability; 

(c) television side-legs and short-haul mes.sage facilities branching off 
backbone routes; 

(d) short-haul message facilities on existing backbone TD-2 or TH 
radio routes; 

* At present, more than 15 million (27 ]>er cent) of tlic Ions distance telephone 
circuit miles anrl moro thftn 60,000 (78 per cent) of the intercity television cir- 
cuit miles of the llcJl System are provided by microwave radio relay. 



THE TJ RADIO RELAY SYSTEM 823 

(o) new facilities to locations where wire construction is difficult; 

(f) alarm, control and order-wire facilities on backbone radio routes; 

(g) general pui'powe local television service; 

(h) bypass service in large city areas where the T13-2 system is at 
capacity. 

2.2 Development Objectives and Performance Characteristics 

Prior to the beginning of the development program, a set of objec- 
tives was specified reflecting the best judgment at that time as to the 
features and capabilities necessary for the new system. These objec- 
tives have been reviewed and modified from time to time us the de- 
velopment of the new system progressed and, in most instances, the 
original reciuirements for the TJ system have Iieen met. 

2.2.1 Frequency Band and Allocation Plan 

The TJ system operates in the 11,000-mc common carrier frequency 
band and provides either message or television service for end-Unk or 
short-haul applications. A frequency allocation plan has been devised 
to permit operation of six two-way TJ channels on the same route 
with common antennas for transmitting and receiving. One-for-onc fre- 
quency diversity, with a simple automatic switch has been provided 
on an optional basis, and little or no cost penalty has been Incurred 
by systems not having such switches. 

2.2.2 Telephone and Television Capacity 

The TJ system has been engineered to transmit 96 channels of ON-2 
multiplex* or 240 channels of L multiplex through nine repeaters for 
a total distance of 200 to 300 miles. The system is also being engineered 
to transmit one monochrome or NTSC color television signal, meeting 
end-link and short-haul objectives over distances of approximately 100 
miles on eacli radio channel. Sufficient video bandwidth has been pro- 
vided to transmit the audio portion of the television program along 
with the video signal on a multiplex basis in the vicinity of mc. 

2.2.3 Order Wire and Alarm 

An order-wire and alarm facility has been provided whi(!h will work 
over the radio for TJ telephone systems or over a wire facility for 
one-way television routes. 



824 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1960 

2.2.4 Stand-by Power 

The staiid-ijy power equipment provides the muiimum discontinuity 
consistent with system cost objective and with other sources of sys- 
tem failure. 

2.2.6 Economics 

One of the primary objectives of this development has been to pro- 
vide system arrangements and operating features at the lowest first 
cost and annual charge consistent with meeting Bell System require- 
ments. Equipment arrangements have been designed to minimize job 
engineering and installation expense. Proper packaging has received care- 
ful consideration. Ease of maintenance is of prime importance in its 
relation to annual charges, and equipment arrangements have been 
devised with this in view. 

2.3 Transmission Objectives 

2.3.1 General 

The TJ hops should be engineered to have a 50-db rms carrier-to- 
noise ratio during periods of free space transmission. The 50-db ratio 
is necessary to provide adequate margin over first circuit noise during 
periods of signal attenuation caused by rainfall. Propagation tests'' con- 
ducted at 11,000 mc indicated that during periods of heavy rainfall 
the attenuation may be in the order of 40 db or more, depending upon 
the length and location of the radio path. Reliable protection against 
selective fades and equipment failure outages can be obtained with 
frequency diversity. Protection against rain attenuation, however, can 
only be assured by engineering sufficient fading margin into the sys- 
tem and by using path lengths appropriate to the particular area of 
the country. 

2.3.2 Telephone 

The TJ telephone end-link objective for cross-modulation and noise 
has been set at 32 dba at the db transmission level point. This figure 
was arrived at by assuming that the random addition (power addition) of 
four such links to a backbone system should not degrade by more 
than 3 db the long-haul, heavy-route system objective of 38 dba. In 
practice, the degradation is not expected to exceed more than about 
1 db in most cases because of shorter end-links and switching losses. 



THE TJ KAUIO RELAY SYSTEM 825 

Due to the large carrier-to-noise ratios expected in TJ systems, all 
of the 32 dha ohjective may he allocated to FM intermodulation and 
noise in tiic Ixiseband circuits. Single-section performance can he de- 
rived from the knowledge of how fluctuation and intermodulation noise 
add as the number of repeaters is increased. For typical values of 
deviation used in the TJ system, the total noise power increase is 
approximately proportional to the numbers of repeaters. 

2.3.3 Television 

The television signal -to- noise* objective for a single-section TJ sys- 
tem is 54 db unweighted. Differential phase and gain objectives for a 
single section are dzl.O degree and ±0.5 db, respectively. 

2.3.4 Stability 

The objective for the short-term net loss variation in a telephone 
channel is le.ss than ±0.25 db, and the long-term net loss variations 
should not normally exceed ±1.5 db. These limits are necessary for the 
system to meet direct distance dialing and other similar Bell System 
requirements. 

III. TRANSMISSION PLAN 

The TJ radio system offers a maximum of six two-way broadband 
channels, each of which provides for either multichannel telephone or 
television transmission. To provide a high degree of reliabihty, only 
three channels are ordinarily used as working channels, the remaining 
three being used for protection on a one-for-one basis, with automatic 
switching at each repeater. 

The radio signals are transmitted to a dual polarized antenna by 
RF chaimelizing and duplexing arrangements. It is expected that most 
systems will use a "periscope" type of antenna arrangement to mini- 
mize the loss associated with long waveguide runs. Such a system uses 
a 5-foot paraboloidal antenna at the base of the tower directed at a 
plane 6- X 8-foot or a "dished" 8- X 12-foot reflector at the top of 
the tower. A 10-foot paraboloidal antenna is available as a direct ra- 
diator for those applications using short towers on natural elevations. 
In addition, an 11,000-mc systems-combining network is available 
so that the TJ system may utilize the horn-reflector antennas installed 
on TD-2 and TH backbone routes. 

A block schematic of a two-section TJ system is shown in Fig. 1. 

* Peak-to-peiik signal to rms noise. 



820 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1960 



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THE TJ llADIO ItELW SYSTEM 



827 



The multiplex and control signals are combined by the high-pass-low- 
pass filter and feed the transmitters of the working and spare radio 
chainiel. At the receiving end, the selected radio receiver feeds into a 
similar lilter combinalicju, which separates the multiplex from the con- 
trol signals. A 2600-cps pilot is transmitted over the system and is 
used to send alarms and orders between the various radio locations. 
The TJ frequency plan is shown in Fig. 2. Because of the expected 
use of the "periscope" antenna system, the plan is based on the use 
of four frequencies for each two-way radio channel. The 10,700- to 



STATION P 



STATION Q 






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Channel 


Transmitter 


Beat Oscillator 


Number 


Frequency, kmc 


frequency, kmc 


Number 


Frequency, kmc 


Frequency, kmc 


4A 


10.715 


10.785 


9B 


11.245 


11.315 


lA 


10.755 


10.825 


12B 


11.285 


11.355 


lOA 


10.795 


10.865 


5B 


11.325 


11.395 


llA 


10.835 


10.905 


8B 


11.365 


11.435 


6A 


10.875 


10.945 


IB 


11.405 


11.475 


7A 


10.915 


10.985 


4B 


11.445 


11.515 


2A 


10.955 


10.885 


IIB 


11.485 


11.415 


3A 


10.995 


10.925 


lOB 


11.525 


11.455 


12A 


11.035 


10.965 


7B 


11,565 


11.495 


9A 


11.075 


11.005 


GB 


11.605 


11.535 


8A 


11.115 


10.045 


3B 


11.645 


11.575 


5A 


11.155 


11.085 


2B 


11.685 


11.615 



Fig, 2 — TJ frpciuency assignment plan. 



828 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 19G0 

11,700-mc common carrier band is divided into 24 channels, each about 
40 mc wide. In a given repeater section, only 12 of these are used, 
resulting in 80-mc spacing between midchannel frequencies. These chan- 
nels are further divided into two groups of six for transmission in each 
direction. The polarization of the channels alternates between vertical 
and horizontal to provide 160-mc separation between signals having 
the same polarization, thereby substantially easing requirements on 
the channel-separation networks. The remaining 12 channel assign- 
ments are used in adjacent repeater sections. These frequencies are re- 
peated in alternate hops. Potential "overreach" interference is reduced 
by reversing the polarization of the third section with respect to the 
first section. Cochannel interference from adjacent repeater stations, a 
necessary consideration in the TD-2 and TH systems because of their 
use of the two-frequency plan, is eliminated in this system by the use 
of the four-freciuency plan. At a given repeater, adequate frequency 
Reparation bet^^"een transmitters and receivers is achieved by using the 
upper half of the band for transmitting and the lower half for receiving. 
This arrangement is inverted at alternate stations. 

The TJ fre(|uency plan and channelizing arrangements permit effi- 
cient use of the entire 11,000-mc common carrier band and establish 
an orderly growth pattern. Additional radio channels may be added 
in the future to a system whose initial requirements are less than its 
maximum capabilities without disrupting service on the working chan- 
nels. Actual route cross sections may vary from a single one-way tele- 
vision system, without protection, up to a full system of three pro- 
tected two-way channels carrymg either telephone or television circuits. 

IV. DESCRIPTION OF SYSTEM 

4.1 General Description 

A basic building block in the TJ system is the transmitter-receiver 
bay. It consists of a frequency-modulated transmitter, a heterodyne- 
type receiver and regulated rectifiers operating from standard ac line 
voltages. A block schematic of the bay is shown in Fig. 3. An incoming 
microwave signal from the antenna system is selected by a channel- 
separation network of the type shown in Fig. 4. This network drops 
the desired channel and permits the remaining channels to pass through 
essentially unattenuated for selection in similar networks on adjacent 
bays. The selected channel is fed to the receiving modulator, which is 
preceded by a bandpass filter providing additional preselection. In the 



THK TJ RADIO RELAY SYSTEM 



829 




+ 200V +600V +400V -400V 6.3V AC 6.3V DC 

_L_t 1 L t L 



BASEBAND 
OUTPUT 



POWER SUPPLY 



AC INPUT 



Fig. 3 — Block schematic of the TJ transmitter-receiver bay. 



receiving modulator the signal is heterodyned with the output of a 
local oscillator to produce a difference or intermediate frequency (if) 
of 70 mc. The if signal is amplified and detected in the receiver to 
provide the original baseband intelligence, which may be applied either 
to the next transmitter or deli\'ered to appropriate terminal equipment. 
In the transmitter, the baseband signal is amplified and applied to 
the repeller electrode of the transmitting klystron. The frequency-modu- 
lated output of the klystron is combined with outputs from adjacent 
transmitters in a channel-combining network similar to the receiver 
separation network. The transmitter outputs are then connected to the 
antemia system which in general will be simultaneously receiving from 



830 



THE BELL SYSTEM TECHXIPAL JOURNAL, JULY 1960 




Fig. 4 — Chiuinel separation-combining network, 

the same direction. Electronic automatic fre(iuency control (afc) is 
provided on the receiver local oscillator to maintain the average in- 
termediate fre(]Liency at 70 mc. On the transmitter, an electromechani- 
cal AFC system keeps the average transmitter frequency at the reso- 
nant frec^uency of a highly stable reference cavity. 

The baseband type of repeater distinguishes the TJ system from the 
more common situation in long-haul microwave systems such as TD-2 
and TH. In these, the signal remains in the frequency -modulated form 
throughout the amplification process at repeaters, and the baseband 
signal is only recovered through the use of special terminal e(|uipment. 



4.1.1 Transmitter Radio Frequency Units 

The radio frequency output from the transmitter is provided by a 
Western Electric 445A klystron de\'eloped specifically for the TJ sys- 
tem. The same tube, illustrated in Fig. 5, is used as the receiver local 
oscillator. Typical operating characteristics in both applications are sum- 
marized in Table I. 

Although the nominal output of the transmitter klystron is 0.5 watt 
when operated in the 2f mode, the actual output is dependent to 



THE TJ IIADIO KELAY SYSTEM 



831 




Fig. 5 — Western Electric 445A klystron. 



Table I — Typical Operating Conditions of WE 445A Klystron 



Resonator voltage 
Resonator current 
Repeller voltage 
RF power output 
Oscillating mode 
Electronic tuning 

range 
Repeller modvilation 

sensitivity 
Cooling 



TraDsmilter 



+600 volts 

65 milliiimperes 
-250 volts 
400 milliwatts (minimum) 

50 mc (minimum) 

0.8 mc/volt (minimum) 

forced air 



Beat Oscillator 



+400 volts 

40 milliamperes 
- 125 volts 

50 milliwatts (minimum) 
3J 

50 mc (minimum) 

1.5 mc/volt (minimum) 
natural convection 



Heater voltage 


6.3 volts 


Heater current 
Rej)ellcr capacity 
Mechanical tuning 


0.9 ampere 

6 mmf (maximum) 

0,5 mc/angular degree 


sensitivity 

Output 


matched to \VR90 waveguide 



832 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1960 







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FREQUENCY IN KILOMEGACYCLES PER SECOND 



11.8 



rig. 6 — ■ Average power output of the Western Electric 445A klystron as a 
function of frequency. 

some extent on frequency. This dependence is demonstrated in Fig. 6, 
in which the average output of a number of production tubes is plotted 
against fre(iueucy. 

Precautions against tube damage by positive repeller voltage have 
been included in both the transmitter and local oscillator circuits in 
the form of a clamping diode between repeller and cathode. 

To reduce the maximum dc voltages on the bay and, hence, to sim- 
plify protective arrangements, the klystron body (resonator) operates at 
600 volts above ground. An insulator between the tube output and its 
mating flange keeps this dc potential off the connecting waveguide. 

The output from the transmitting klystron feeds a waveguide net- 
work, which serves the dual purpose of an afc discriminator and power 
monitor. A schematic of the network is shown in Fig. 7. The first di- 
rectional coupler feeds a waveguide hybrid, which, in conjunction with 
an invar reference cavity and a pair of silicon diodes, forms an rf 
discriminator. The operating principles of the discriminator have been 
described by Pound,^ and a typical output characteristic is shown on 
the schematic. Zero output at the "tails" of the discriminator character- 
istic is controlled by the balance control, while the crossover point can 
be set to any frequency in the TJ band by tuning the reference cavity. 
The slope of the discriminatoi" characteristic is determined by the loaded 
Q of the reference cavity, which is nominally 900. The second directional 
coupler in the waveguide network monitors transmitter power output, 



THE TJ RADIO RELAY SYSTEM 



833 



TO 
ISOLATOR 



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COUPLER 



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DIRECTIONAL 

COUPLER 



WAVEGUIDE NETWORK 



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FREQUENCY IN 
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'-•-'transmitting 
I klystron 

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Fig. 7 — Schematic of the transmitter afc discriminator and power-moiiitor- 
ing network. 



and, with a nominal +27 dbm signal from the klystron, the coupler out- 
put is +3 dbm. Normally, this signal is fed to a detector and meter cir- 
cuit that is calibrated in decibels referred to the +27 dl)m level. For 
test purposes, the detector can be rapidly removed, permitting frequency 
and other transmitter characteristics to be checked. 

To minimize the nonlinear effects produced by reflections m the an- 
tenna feed, a high-performance ferritc isolator is required between the 
klystron output and the antenna system. The field displacement isolator" 
shown m Fig. 8 connects to the output port of the afc waveguide net- 
work, and typical forward and reverse loss characteristics are shown in 

Fig. 9. 

The transmitter output is fed to the channel-combunng networks 
through a waveguide switch, which normally allows the rf energy to 
pass through unattenuated. AVhen a klystron is replaced, the mitial os- 
cillating fre(iuency may be considerably different from nominal. To pre- 
vent intei-ference with adjacent channels, the switch can be temporarily 
closed, thereby attenuating the output signal by more than 80 db. 



834 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1960 




Fig. 8 — Field displacement isolator. 



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FREQUENCY IN KILDMEGACYCLES PER SECOND 



Fig. 9 — Forward and reverse loss characteristics of the TJ isolator. 



THE TJ RADIO RELAY SYSTEM 



835 



The channel-combining networks are identical with the channel-sepa- 
ration networks on the rceeivcM- (Fig. 4). The principle of operation has 
been described elsewhere, '° and representative transmission characteris- 
tics ai'e given in Fig. 10. 

4.1.2 Receiver Radio Frequency Units 

Incoming rf channels from the antenna are selected by channel- 
separation networks identical with the combining networks mentioned 
in the previous section. In the case of the last receiver in a line-up, the 
separatnig network is not retiuired, since at this point the number of 
RF channels has been reduced to one. 

The selected channel is fed to the receiver modulator through a band- 
pass filter, a waveguide tuner and a critically dimensioned waveguide 
spacer. The filter serves the two-fold purpose of providing suppression 
against interfering image signals and enhancing the efficiency of the 
modulator by reflecting out-of-band modulation products back into the 



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DEVIATION FROM CENTER FREQUENCY IN MEGACYCLES PER SECOND 



Fig. 10 — Representative traiismisaion charm; teris tics of llie rf channel separ- 
iition-coiuhiiiing networks. 



836 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 19G0 



60 



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FREQUENCY FROM MIDBAND (N MEGACYCLES PER SECOND 

Fig. 11 — Representative transmission characteristics of tlie Rf receiver band- 
pass filters. 



converter in the proper phase. Correct phasing is achieved by choosing 
a suitably dimensioned waveguide spacer, which determines the electrical 
path length traversed by the modulation product. To minimize reflec- 
tions in the waveguide run between the antenna and the modulator 
input, the modulator input impedance must be closely matched to the 
waveguide impedance. This match is optimized by the adjustment of 
a two-stub tuner located between the bandpass filter and the modulator. 

The bandpass filters are of two types, having either three or four 
resonant cavities. The additional selectivity of the four-cavity filter is 
required when a receiver is not provided with a channel-separation net- 
work. Typical transmission characteristics for both filter types are shown 
in Fig. 11. 

The balanced receiving modulator is shown schematically in Fig. 12. 
Waveguide inputs are provided for both the incoming and local oscilla- 
tor signals, while a coaxial output connection is provided for the 70-mc 
IF signal. The unbalanced output is obtained by reversing the polarity 
of one diode relative to the other in the hybrid junction assembly, thus 
permitting paralleled unbalanced output connections. 



THE TJ KADIO llELAY SYSTEM 



837 



RF INPUT 

RETURN LOSS: 

> 30 DB AT fo 

> 22.5 DB AT fn ± 10MC5 



MODULATOR 



BANDPASS 
FILTER 



^■>^ 




BE AT- OSCILLATOR 
MONITOR 



TEFLON 
"l INSULATOR 



OVER-ALL LOSS (MAX) RF INPUT TO IF OUTPUT = 9.0DB 
MAXIMUM NF AT TUNER INPUT = 15,0DB 



Fig. 12 — Schematic of the balanced receiving modulator. 

The local oscillator is a Western Electric 445A klystron identical with 
that used in the transmitter. It feeds the modulator through a monitor- 
ing coupler and an attenuator, which can be adjusted to give a power 
input to the modulator of approximately dbm. 



4.1.3 Intermediate Frequency Units 

The IF output from the receiver modulator is amplified in a preampli- 
fier followed by the if main amplifier. These units have a passband 
centered at 70 mc with typical gain and delay characteristics illus- 
trated in Fig. 13. Input and output impedances are 75 ohms unbalanced 
with a minimum return loss of 20 db. Table II summarizes the gam and 
tube complement for both units. 

The main amplifier has a nominal power output of -1-5 dbm, and in- 
ternal automatic gain control on the first six stages maintains the out- 
put within 4 db of nominal for a 40-db change in input level. Matched 
double-tuned coupling circuits are used in all if units, with nonadjust- 
able auto transformers in the interstage networks. The only adjustable 



838 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1960 
1.0 

en 0.5 

_r 

m 
a 

2-0.5 

z 

< -f.O 

15 

y-,.5 

o 

r«- 

S -2.0 

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a: 

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o 

^-3.0 

> 

ui 

Q -3.5 



u 

WD 

a_i 
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oo 

SO 



■4.0 
40 

30 

20 

10 



























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■^ 


^ 




.^ 


N. 










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/ 












\ 




















\ 




















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1 










































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S^ -10 





\, 
















' 






s 


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L 


NEQL 


ALIZ 


■/ 










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s. 








/ 










\ 


^ 


s 


^, 




/^ 


h^ 1 


/ec 


UALIZ 


ED 























SO 



54 58 62 66 70 74 76 62 66 90 94 

FREQUENCY IN MEGACYCLES PER SECOND 



Fig. 13 — IF gain and delaj' characteristics. 

elements are in the input and output coupling networks, which are ad- 
justed for maximum return loss. The marked reduction in the immber 
of tuning adjustments simplifies testing and maintenance, while reduc- 
ing the possibility of maladjustment. Alternative couphng networks 
providing higher gains per stage were considered in the initial design 



Table II 



Unit 


Gain 


Tube Complement 


Preamplifier 
Main amplifier 


32 dh 
75 db 


two 417A triodes 

two 435A tetrodea 

seven 435A tetrodes 



THE TJ RADIO RELAY SYSTEM 839 

but were discarded in favor of the matched circuit, which is much less 
sensitive to tube changes. 

In the interests of minimizing maintenance aud reducing the number 
of tube types, the Western Electric 435A is used wherever possible. 
This tube is a tetrode of proven integrity with a life expectancy based 
on field experience of 50,000 hours. 

The plate supply for the preamplifier and main amplifier is +200 volts. 
Th(^ total plate current for all tubes in the main if amplifier is fed through 
a common resistor located in the power supply. Since the amplifier 
gain, and hence the total plate current, is automatically adjusted to 
compensate for changes in received signal level, the voltage across the 
common resistor is a measure of the received signal strength. This 
voltage is used to actuate the comparator circuit in a diversity system. 

The 75-ohm unbalanced output from the main amplifier feeds the 
Umiter-discriminator circuit. Plate limiting is employed in a three-stage 
circuit, using 435A tetrodes. Low-forward -impedance gold-bonded diodes 
provide the clipping action and give a total dynamic limiting of approxi- 
mately 40 db. The discriminator uses two separately driven antircsonant 
circuits tuned to approximately 52 and 87 mc. When these circuits are 
driven by the fi-e(iiiency-modulated signal, amplitude modulation re- 
sults, which is detected by a double diode and fed through video cou- 
pling circuits to the balanced receiving baseband amplifier. 

4.1.4 Baseband Units 

A maximum peak-to-peak signal of 10 volts is required to modulate 
the repeller of the transmitting klystron, to provide ±4 megacycles 
deviation. This is provided by a two-stage, balanced video amplifier 
with an optional input impedance of 75 ohms unbalanced or 124 ohms 
balanced. The transmitting amplifier utilizes four Western Electric 437A 
triodes and has a nominal voltage gain of 37 db, adjustable over a range 
of ±3.5 db. A peaking circuit in the amplifier input provides low-fre- 
quency phase correction for a complete repeater when it is used for 
television transmission. The peaking circuit is provided as a wiring op- 
tion and is not used in telephone applications. 

The receiver baseband amplifier is a single stage balanced circuit 
utilizing Western Electrit- 417A triodes. lis principal function is to con- 
vert from the relatively high output impedance of the discriminator to 
the line impedance. This may be 124 ohms balanced or 75 ohms un- 
balanced in accordance with the wiring option cho.sen within the imit. 
With an FM deviation of 8 mc peak-to-peak, the discriminator-balanced 



840 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1U60 



amplifier combination gives a minimum balanced output of —1 dbm 
(-Idbv). 

4.1.5 Automatic Frequency Control 

Automatic frequency control is used on both the transmitting klystron 
and the receiver local oscillator. In the former case the control is electro- 
mechanical; in the latter, it is electronic. 

The transmitter frequency-error signal is derived from the rf dis- 
criminator network described in Section 4.1.1. This error signal is applied 
to the AFC circuit shown in functional schematic form in Fig. 14. When 
the frequency error exceeds 300 kc, the meter-type relay operates one of 
the mercury relays, causing the drive motor to mechanically tune the 
klystron cavity in a direction to reduce the error. The drive motor then 
continues to operate until the meter relay is reset. If the frequency error 
still exceeds 300 kc, the same sequence of events will be repeated mitil 
the error is less than 300 kc. The accuracy of the afc system is con- 
trolled by the stabihty of the reference cavity in the ef discriminator 
circuit. Over the ambient temperature range from —20 to +120°F the 
transmitter frequency is maintained within ±0.03 per cent of its nomi- 
nal value. 

Electromechanical control of the transmitter frequency is preferred 
to electronic control, since most of the frequency error results from 
changes in ambient temperature. Mechanical tuning corrections for this 
type of fre(|uency error have much less effect on modulation linearity 
than does a corresponding correction produced electronically. 



METER-TYPE 
RELAY -■, 



TO RF 
DISCRIMINATOR" 



CONTROL WINDING- 



DC SUPPLY -• 



— PHASE - 

splitting 
^"■capacitor 



GEAR RATIO 
1875:1 




Fig. 14 — Functional schematic of the transmitter afc circuit. 



THE TJ RADIO BELAY SYSTEM 



841 



Automatic frequency control on the receiver is performed electronic- 
ally silica, in this case, the beat oscillator operates at a fixed frequency 
and modulation linearity is not of importance. The if error voltage is 
derived from the signal discriminator, as illustrated in Fig. 15. The 
error signal is ampUfied and applied m series with the beat oscillator 
klystron repcUer voltage with the appropriate sign to reduce the error. 
This automatically maintains the intermediate frequency at the cross- 
over frequency of the discriminator. If the cross-over point is not exactly 
at 70 mc, an adjustable bias is provided at the input of the afc ampli- 
fier, which permits adjusting the intermediate frequency to exactly 70 
mc. To limit the frequency excursions of the beat oscillator and prevent 
the receiver's locking onto unwanted signals, a clamp circuit is mcluded 
in the afc loop. An additional clamp circuit for the protection of the 
klystron beat oscillator ensures that the magnitude of the negative 
repeller voltage never drops below 40 volts. 

The frequency of the receiver beat oscillator may be above or below 
the incoming signal frequency, depending on the channel number. These 
two conditions correspond to a phase reversal at the discriminator out- 
put, and must be compensated for to ensure stability in the afc feed- 
back loop. The phase correction is obtained by reversing the balanced 
connections between the limiter-discriminator chassis and the receiver 
baseband-AFc unit. 



VIDEO OUTPUT 
- TO BASEBAND 
AMPLIFIER 




FJK. 15 — Functional schematic of the receiver afc circuit. 



842 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1960 



4.1.6 Power Supplies^'^ 

All dc voltages and ac filament supplies are derived from an ac-oper- 
ated power supply in the lower third of the transmitter -receiver bay. 

The principle of operation is illustrated in the block schematic of Fig. 
16. Only the —400 volt output is directly controlled by the feedback 
loop ; regulation on the other outputs is dependent on their tracking the 

— 400 volt output. This is achieved by suitable design of the filter net- 
works and the rectifier regulation characteristics. A sensing relay on the 

— 400 volt output ensures that negative repeller voltage is applied to 
the klystrons before the positive resonator voltages. 

Silicon diodes are used thi'oughout as the rectifying elements, so that 
the only active devices in the circuit arc two Western Electric 310A dc 
amplifier tubes and two voltage reference tubes (423A and 427 A) in the 
feedback control loop. Compared with the more usual series tube regula- 
tion, it is expected that the combination of solid state rectifiers and long- 
life control tubes of proven integrity will greatly reduce power consump- 
tion and the annual charges associated with the maintenance of the series 
tube type of regulated power supply. 

The stability of the dc output voltages over the ambient temperature 



SEQUENCING 

CONTACT 




o +6D0V,75MA 



o + 400V, 50 MA 



< 15 MV 

P-P 
RIPPLE 



■I- 200V, 400 MA 

6.3V, 0.9A 

TRANSMITTING KLYSTRON 

6.3V, 0.9A 

BEAT OSCILLATOR 

KLYSTRON 



■o 6,3VAC,B.ZA 



-400V, MA<25MV 
P-P RIPPLE 

-^ -400V 

SENSING RELAY 



DC AMPLIFIER 



REFERENCE TUBE 

Fig. 16 — Power supply bloek schematic. 



THE TJ RADIO RELAY SYSTEM 



843 



am 

(TuJ 
CCiD 



4 
































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^ — \ — 


1 1 















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^ 
^ 


^ 


■-MESSAGE 




1 1 


M 














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-fl 





























4 6 8 10 20 40 60 BO 100 200 400 600 10001 2 4 6 8 10 

FREQUENCV IN CYCLES PER SECOND MEGACYCLES PER SEC 

Fig. 17 — Gain-frequency characteristic of a TJ repeater. 

range 32° to 120°F and with line voltage variations of ±10 per cent is 
better than ±0.5 per cent. 

4.1.7 Omr-All Performance Characteristics 

The gain-frequency characteristic of a single TJ repeater relative to 
1 kc is shown in Fig. 17. The absolute gain of a repeater is nominally 16 
db measured from transmitter input to receiver output between 124- 
ohm balanced impedances. In diversity applications, 6 db of this gain is 
lost when two traiismitters arc fed simultaneously rcsultuig in a net gain 
of 10 db lietween HP FL IN and HP FL OUT on the diversity switch 

unit. 

To illustrate the linearity of the T.T equipment, Fig. 18 gives the re- 



50 
§45 



> 40 



35 



3 30 



< 15 









NOISE-LOADING SPECTRUM 60-1052 KCS 
NOISE MEASURED IN 1002 KCS SLOT 
EQUIVALENT MESSAGE CHANNELS - 240 
REFERENCE INPUT = -25 DBM AT 
TRS INPUT (-33TL) 




> 








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12 Lir 


KS 




N 




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/ 












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-25 -20 -15 -10 -5 O 5 to 

OB FROM REFERENCE INPUT 



Fig, 18 — Typical noise loading performance of a TJ system. 



844 



THE BELL SYSTEM TECHIVICAL JOURNAL, JULY 1900 



suits of noise loading tests on a system between Mt. Clemens and Port 
Austin, Michigan. The system was measured with different numbers of 
links in tandem, and the results are presented for one and 12 links. The 
single-link data are the average performance on six individual links; the 
data on 12 tandem links include these six links in the over-all measure- 
ment. For test purposes, 240 channels of single-sideband suppressed- 
carrier multiplex were simulated by a band of white noise, and the 
fluctuation and intermodulation noise was measured in a slot at the top 
end of the transmitted band. The db reference level corresponds to a 
total input noise power of —25 dbm at HP FL IN on the diversity switch 
and transmission unit, which is a —33 db transmission level point. 

The contribution of fluctuation-type noise to total noise at the receiver 
output will depend on the received rf carrier level. This relationship is 
illustrated in Fig. 19, where the noise is given in dba for the top channel 
in a system carrying 240 channels of suppressed-carrier type-L multiplex. 
The dba readings are based on a peak-to-peak frequency deviation of 8 
mc and a load capacity rating for 240 channels of 21 db. 

In television transmission, the Imearity characteristics of interest are 
differential gain and phase. Without pre-emphasis or delay equalization, 
the differential gain and phase on a single link are respectively 0.5 db 
and 4.5°. With 13.2 db of pre-emphasis and if delay equalization, the 
corresponding figures are 0.1 db and 0.5*^. 

Low-frequency noise objectives at the output of a TJ receiver require 




-30 -40 -50 -eo -70 -so -90 

RECEtVER CONVERTER INPUT LEVEL IN DBM 



Fig, 19 — Fluctuation noise output as a function of received signal level. 



THE TJ BADIO RELAY SYSTEM 



845 



the use of dc hoiitcrs on the transmitter and local oscillator klystrons. 
The ratio of peak-to-peak signal to low-frequency unweighted rms noise 
at the receiver output on one TJ link is 67 db. 

4.2 Diversity Switching Arrangements 

4.2.1 General 

Most applications of the TJ system will use one-for-one frequency 
diversity. This provides protection against multipath fading and equip- 
ment failures, and provides alternate facilities during maintenance and 
equipment additions. At each repeater the diversity equipment selects 
one baseband output from two receivers and applies this signal simul- 
taneously to two transmitter.s. The selection of a receiver is controlled 
by a logic circuit, which, along with the switch, is contained in the diver- 
sity switch and transmission unit. 

4.2.2 Diversity Switch and Transmission Unit 

Fig. 20 is a block schematic of the diversity switch and transmission 
miit as used at a telephone repeater. The audio and high-fre(iuency com- 
ponents of the baseband signal are separated and subsc(|uently recom- 
bined by high-pass-low-pass filters. This permits connection to the 
D-type alarm and order-wire equipment without affecting through trans- 



r 



SINE-WAVE POWER 

FOfi BMC PEAK-TO -PEAK' 

DEVIATION 



-2DBM 
(SWITCH INPUT) 



RECEIVER I 



COMPAR- 
ATOR 



J^ 



RADIO 
RECEIVER 



-^ 



PASSIVE 
RECEIVER 

PILOT 
MONITOR 



LOGIC 
CIRCUIT 



AUTOMATIC SWITCH 



I RADIO 

-IB DBM TRANSMITTER 
(SIGNAL / W /^ 

-12 DBM OUTPUT) / / 

(HIGH-PASS I / |-H ~ 

FILTER INPUT) h— i 



TO D-TVPE 

ALARM 

AND 

MULTIPLEX 



^ 



TRANSMITTER W 
PILOT MONITOR 



1 



RADIO 
TRANSMITTER 

Z 



TRANSMITTER Z 
PILOT MONITOR 



Fig. 20 — Block schematic of the diversity switch and transmission unit 



846 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1960 



mission of the higher frequency multiplex signals. Furthermore, if the 
multiplex signals are connected through dropping equipment that does 
not normally pass low frequencies, the separation filters enable the audio 
portion of the band to be bypassed without undue attenuation. Typical 
loss-fre<iuency characteristics for the split-apart filters are given in Fig. 
21. Adjustalilc attenuators and pads provide for equalizing receiver levels 
and setting frequency deviations on the transmitters. 

On a fully developed TJ route the order-wire and alarm information 
will be carried on only one of the three diversity pairs. In general, there- 
fore, split-apart filters are not required on switch units associated with 
the second and third systems. In these cases, the switch output can be 
connected directly to the Y-pad input through a 10-db pad. An excep- 
tion to this rule occurs with multiplex dropping facilities, as described 
previously. 

The signal switch is controlled by a logic circuit operating under 
instructions from transmitting and receiving pilot monitors and an rf 
signal comparator. In general, three monitors are utilized to sense the 
presence of a 2000-cps pilot tone at the two transmitter afc discrimi- 
nators and at the output of the idle receiver. The pilot tone is applied 




0,4 0.6 1.0 2 4 6 8 10 20 

FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 21 — Typical loss-frequency characteristics of baseband split-apart filters. 



THE TJ RADIO RELAY SYSTEM 



847 



continuously from one end of the system and is looped around at the far 
end terminal to provide pilot tone on the return path. The 2()00-cps 
tone is also used for signaling purposes, and alarm functions are per- 
formed liy interrupting the tone temporarily. This arrangement pro- 
vides a fail-safe alarm system, which will be described in more detail 
in Section 4.4. 

The functional schematic of a pilot monitor and its selectivity charac- 
teristic are shown in Fig. 22. During signaling, the pilot tone is applied 
to the line intermittently, and enough time delay must be built into the 
monitors so that the absence of tone between pulses may be ignored. 

The RF signal comparator is actuated by voltages proportional to the 
IF plate currents in the X and Y receivers. As described previously, these 
voltages are an indirect indication of the received kf signal strength. 
Fig. 23 shows a simpHfied schematic of the comparator, which provides 
an output to the logic circuit in the form of high and low contacts on the 
comparator relay c. Diodes ckI and cr2 act as clamps on the comparator 
tube grid voltages, so that the circuit is inactive until one or other of 
the received rf signals reaches a level of approximately —40 dbm. The 
operational characteristic of the comparator as a function of input signal 



SEALED MERCURY 
CONTACT RELAY 




2600 CPS 

BANDPASS FILTER 

(NOT USED ON TRANSMITTER 

MONITORS AT NEAR-END 

TERMINALS) 



OPERATE TIME: PASSIVE = 580 MS 
ACTIVE =360 MS 

RELEASE TIME: PASSIVE = 1000 MS 
ACTIVE = 2290 MS 



ALARM 
LEADS 



.t'-io 



RESPONSE 
RELATIVE TO 
BAND CENTER 




-400 -300 -200 -100 100 200 300 400 
DEVIATION FROM NOMINAL 2600 CPS IN CPS 



Fig. 22 — Functional Hchemiitic iiiid selectivity characteristic of the 2600-cps 
pilot mouitor. 



848 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1960 

BALANCE 




IF B + 
RECEIVER X 



1 + 200VCX) 



+200VCY3 



RECEIVER y 




20 
20 40 60 80 100 

RF INPUT RECEIVER (X) IN DECIBELS 



Fig. 23 — Schematic and operatiug characteristic of received signal comparator. 



levels to the two receivers is also indicated in Fig. 23. Both the pilot 
monitor circuits and comparator are designed to fail-safe in the sense 
that a failure will bring in an alarm or at least prevent a switch to a bad 
channel. 

When the diversity switch is used with a television system, the 2600- 
cps pilot tone cannot be used, since it lies within the video band. Like- 
wise, the band-separation filters cannot be used. As a result, the diversity 



THE TJ RADIO RELAY SYSTEM 849 

switch is controlled entirely by the comparator relay, protecting the 
system against selective fading but not against all equipment failures. 

4.2.3 Diversity Switch and Logic Circuit 

The signal-switching function is performed by a wire-spring relay with 
make-before-break contacts. During switchover, the relay contacts are 
momentarily bunched (for approximately one millisecond) paralleling 
the outputs of the two receivers. If the receiver outputs are equal in 
magnitude and phase, connecting them in parallel produces no level 
change, since they are at the same potential. With selective fading the 
condition of etiual baseband levels during a switch is generally met, 
since the fade must be quite deep before it affects the receiver output. 
Thus, with proper adjustment, switches due to selective fading are hit- 
less and will not result in data transmission errors. Likewise, manual 
switches made during system maintenance will also be hitless. 

In the case of switches resulting from an ec|uipment failure, the situa- 
tion is different. Due to the time delay built into the pilot monitors, 
2.3 seconds are required to recognize the trouble and make the switch. 
During this period there will be no transmission and errors in data 
systems will result. This is not an extremely serious situation, since 
equipment failures will be much less frequent than atmospheric disturb- 
ances. 

Each diversity switch can be operated from the control center by 
means of an order from the D-type order-wire and alarm equipment. 
This feature is extremely useful for maintenance purposes and permits 
the rapid location of any level changes that might occur on a system in 
service. 

4.2.4 Squelch Circuit 

If a complete power failure should occur at a TJ repeater station, 
the loss of transmitted carrier will cause an increase in noise output from 
receivers at adjacent stations. This could result in the alarm equipment 
being unable to interrogate the system to locate the station in trouble 
and might require a visit to every repeater on the route. Furthermore, 
if the system connects to a long-haul system carrying other circuits, 
the excessive noise could make the other circuits uiuisable. To eliminate 
this possibility, a squelch circuit is available that cuts off the receiver 
baseband ampUfier when the received kf signal drops below a prescribed 
value. The circuit is actuated by the if plate current in the same manner 



850 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1960 

as the comparator. When the plate current exceeds a certain value, plate 
voltage is removed from the receiving amplifier tubes. The depth of fade 
at which the squelch circuit operates is adjustable, but is generally set 
to actuate at a received rf level of —80 dbm. _ .^ 

4.3 Nondiversity Applications 

Situations will arise where diversity operation is not required, most 
commonly in te;levisi(Ki applications. To facilitate interconnection be- 
tween transmitters and receivers when the diversity switch and trans- 
mission unit is not required, a special connecting panel is available to 
provide the necessary attenuators, equalizer, cable terminations and 
access jacks. 

4.4 Order-Wire, Alarm and Control System* 

Tlie TJ repeater stations are usually unattended. For this mode of 
operation to be feasible, attended points responsible for the maintenance 
of the system must have information promptly as to equipment failures 
and other abnormal conditions occurring in the unattended radio sta- 
tions. The type-D alarm, control and order-wire system was developed 
for this purpose. As the name implies, this system also provides the 
voice-communication facility between stations and remote control from 
an attended point of certain functions at the unattended stations. 

The principal features of the D2 system may be summarized as fol- 
lows : 

i. It transmits up to 18 distinct alarm functions and six other indica- 
tions from each unattended station to its associated alarm center. The 
indications arc generally used to determine which receiver of a diversity 
pair is in use. 

ii. It transmits 11 remote control orders from the alarm center to each 
unattended station under its control. 

iii. It allows a maximum of 14 unattended stations to be associated 
with one alarm center "main station," 

iv. It provides order-wire talking and monitoring facilities at each 
radio station and at the alarm center, with extensions to other points 
as required. The order-wire channel between radio stations is transmitted 
in the baseband below 4 kc. All points having access to the circuit are 
linked together on a multistation or "party line" basis. 

V. It provides alarm and control signaling by means of a single fre- 
(juency tone of 2000 cps within the same four-wire circuit used for the 
order wire, so that no line facilities other than the voice-frequency chan- 

* This section prepared by H. H, Haas. 



THE TJ RADIO lUCLAY SYSTEM 851 

nel of the radio system are required, except for extensions to points off 
the radio system. 

4.4.1 Alarms 

The alarm center (main station) signals the remote points (sub- 
stations) by pulsing the 2600-cps tone. Signal receivers, bridged on 
the outgoing line at each substation as shown in Fig. 24, transform these 
tone pulses into dc pulses to operate decoding relays. Each substation is 
capable of recognizing pulse codes directed to it and translating these 
codes into orders or, alternatively, answering codes that are essentially 
queries by reverting certain received pulses. 

The main station transmits a continuous tone in the idle condition 
through each intermediate substation. The normally closed pulse-revert- 
ing path of a terminal substation sends the tone back to a signal receiver 
at the main station. This closed loop is self-alarming in the event of 
failure of the alarm line and, in addition, provides the means of alerting 
the main station when a trouble occurs at any substation. 

When an alarm condition occurs, substations initiate automatic in- 
terrogation from the main station by inserting a 2000-cps stop filter 
in the return line mitil this process is completed. A relay sending direc- 
tor ('ircuit at the main station reacts to the interruption of the tone loop 
by sending out a train of pulses to successively identify the substation 
at which the alarm occtu'red and the nature of the trouble — that is, 
which of the 18 alarms exist at that station. The substation provides 
automatic identification and scanning by reverting a unique combina- 
tion of the rec(!ivcd pulses, and momentarily closes the pulse-reverting 
path from outgoing to return line at appropriate times, under control of 
the relay circuit that counts and decodes the received pulses. The in- 
formation derived from re\'erted pulses recei\'ed at the main station is 
displayed on lamps. An attendant at the alarm center can also scan the 
alarms at any time by sending out an order. 

4.4.2 Remote Controls 

Basically, remote control techni<iues of the D2 system are similar to 
those involved in reporting alarms. Typical orders to a radio station may 
be for remote manual control of the emergency engine alternator or for 
protection switching. Other orders are provided to retjuest a scan of 
alarms and other indications. In the fir.st example, one-way selective 
signaling is employed; in the second, revertive pulsing conveys the re- 
sponse to the interrogation. 

The sending director sends out the station-selection and order code 



852 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1960 



UJUJ 



QlU 

tVjUJ 






(EJ-IZq- 












Will 



o 



^i-a(t>3 



m 



E <iija 



..It la i_ 



5^ 



l-OS 

01 



pz 



< > m 



z 
o 

i\ 

(E 

UJ 



in 



u 



CD LU 

«oz 



S3 



<« 



zE 

OK 

15 



ffl 



bio 



THE TJ RADIO RELAY SYSTEM 853 

pulses in response to the operation of keys. To send an order the alarm 
center, an attendant first operates one of 14 station keys, then one of 
1 1 order keys. The relays at the substations count and decode the pulses. 
The wanted substation recognizes from the initial pulses that the order 
applies to it, and proceeds to translate the remaining pulses into one of 
11 orders at that particular station. 

The 2600-cps tone-signaling equipment of the D2 system is a transis- 
tor version of the standard in-band type used on telephone trunks.^^ In 
the idle "tone-on" state, when no alarm or control pulsing is taking place, 
the tone level is low and is filtered out of the monitoring circuits of the 
order wire. The pulsing setiuence, although at a higher level, is of such 
short duration that it can be transmitted over the order wire without 
objectionable interference with voice communication. Conversely, in- 
terference between the voice and signaling equipment is prevented by 
guard circuits built into the signaling equipment. 

Main stations may be located at the focal point of up to four con- 
verging radio routes or at an intermediate point along a route as well as 
at the end of the route as shown in Fig. 24. In addition, a main station 
can be used to alarm radio routes that converge at unattended substa- 
tions (junction substations). 

4.4.3 Order Wire 

Way stations on the order wire talk on one side of the four-wu-e line 
and monitor on the other. To complete the circuit, the two sides are 
bridged at the main station so that all intersubstation communication 
is via the main station. 

4.5 Antenna Systems 

Some of the factors afTccting the choice of antennas are; antenna 
height required, gain, return loss, location and cost. Generally speaking, 
there are five standard antenna arrangements suitable for use with the 
TJ system. 

4.5.1 5-Foot Paraboloid 

The antenna shown in Fig. 25, is a rear-feed, ring-focus, dual-polarized 
paraboloidal antenna. It comprises a 5-foot-diameter paraboloid, a pri- 
mary feed system, a radome and a ball-swivel base with adjustable 
support rods. It is primarily intended for use in "periscope" antenna 

systems, but may be used as a direct radiator. 

The feed is made of a straight section of circular waveguide, which 



854 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1960 




Fig. 25 — TJ 5-foot dual-polarized paraboloidal antenna. 

passes through tlie apex of the paraboloid and along its axis. A specially 
shaped 3-inch disc reflector is located at the waveguide aperture and 
serves to direct the energy back on the surface of the paraboloid. 

The gain of this antenna is 42.1 db over an isotropic radiator at a 
fre<iuency of 11,200 mc and varies less than 1 db over the 10,700- to 
11,700-mc connnon carrier l)and, as shown in Fig. 20. The return loss 
and cross-polarization discrimination are in excess of 20 db. Fig. 27 shows 







47.8 



10.6 10.8 11.0 11,2 11.4 11.6 11.9 

FREQUENCY IN KILOMEGACYCLES PER SECOND 



Fig. 26 — Gain-frRf|UGncy rluiriu'teristic of the TJ radio 5- !ind 10-foot anteniuis, 



THK TJ RADIO RELAY SYSTEM 
230' 220° 200° 180° 160° 140° 



855 



130° 



240' 




330° 3-40° 350' 10° 20° 30° 

Fig. 27 — Typical 360° nidiiitinii pattern for the TJ 5- niul 10-foot antennas. 

a typical 3G0° radiation pattern of the 5-foot paraboloidal antenna, and 
Fig. 28 shows an enlargement of the ±15 degree direct and cross-polar- 
ized patterns. 

4.6.2 10-Foot Paraboloid 

Natural elevations are often available in mountainous comitry for 
repeater sites. In these cases, path clearance is not usually a problem 
and the antennas are mounted on short towers whose height is just 
sufficient to provide foreground clearance. The antenna is composed of a 
10-foot paraboloidal refloetov, a primary feed system and an A-type 
mounting frame that provides azimuth and elevation angle aiming ad- 
justments. The feed arrangement is similar to the 5-foot paraboloidal 
antenna. The 10-foot paraboloid does not have a radome, but heaters 
are available for both the feed and the paraboloidal reflecting surface. 



856 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1900 




-12 -8 -4 4 8 12 

AZIMUTH IN DEGREES FROM MAJOR LOBE 

Fig. 28 — ±15" radiation pattern of the TJ 5-foot antenna. 

The gain of the antenna is approximately 48 db for the two polariza- 
tions as shown in Fig. 26. The return loss and cross-polarization discrim- 
ination are in excess of 20 db. Fig. 27 shows a typical 360° radiation 
pattern of the 10-foot paraboloidal antenna, and Fig. 29 shows an en- 
largement of the ±15 degree direct and cross-polarized patterns. 

4.5.3 Periscope Antennas 

In many areas of the country the terrain is relatively fiat and paths 
25 to 30 miles in length require antenna heights in the order of 250 feet. 
The use of paraboloidal antennas as direct radiators is not desirable, 
because of the long rectangular waveguide runs and their associated high 
loss. The horn-reflector antenna, with its 3-inch circular waveguide and 
combining network performs well at 11,000 me, but it is too expensive 
The "periscope" antenna system, which satisfactorily fills this need, 
consists of a paraboloidal antenna mounted at or near ground level, 
usually on the roof of the repeater station, illuminating a reflector at the 
top of the tower. It has the advantages of requiring a minimum length 
of waveguide and providing an over-all antenna system gain that is 
equal to or greater than the gain of the paraboloidal antenna alone. 

Some of the factors determining the gain'' of a "periscope" antenna 
are the frequency, the relative size of the apertures of the paraboloid 
and the reflector, and the separation between them. Standard arrange- 
ments for the TJ system consist of the 5-foot paraboloidal antenna 



THE T.r RADIO RELAY SYSTEM 



857 



?S 30 
D 

U.1U 40 

N 

I* 

° 50 



70 
-16 







- — 


I 














DIRECT 
IRESPONSE 












r 


A 














/ 


' 


s. 










/ 


^1 
1 




\V 








/^ 


/ 
/ 






\ 
\ 

\ 

\ 


\ 


\ 


/ 




/ 

/cROSS-P 
/ RESP( 
/ 


3LAR1ZED 
)N5E 




\ 


. 




* 











-12 -B -4 4 8 12 

AZIMUTH IN DEGREES FROM MAJOR LOBE 



Fig. 29 — ±15° radiation patten of the TJ 10-foot antenna. 

and either a 6- X 8-foot plane reflector or an 8- X 12-foot reflector, 
which may be plane o)- curved. Fig. 30 shows the approximate gains to 
be expected for these TJ "periscope" antennas. Estimated radiation pat- 
terns of the 6- X 8-foot plane and the 8- X 12-foot curved reflector are 
shown in Fig. 31. 



< o 

UJ H 



46 



40 





/ 


^ 


1 
8'X 1 


2' 
ED 
















/ 




r 


\ 


\, 














/ 







e' X 1 

PLAN 


p-^> 


V 


k 












.-7 






: 


\ 


\ 


\ 


V 








/ 


6' 


^>s 












\ 


\ 






/ 


PL 


ANE 














\ 


\ 


^ 



100 150 200 250 300 400 500 600 700 

SEPARATION BETWEEN ANTENNA AND REFLECTOR IN FEET 



Fig. 30 — Gain of periscopic anteuna system. 



858 



THE HELL SYSTEM TECHNICAL JOURNAL, JULY 19G0 



O 4 



S 12 
o 



•n 

UJ 

m IS 

u 

'='20 



22 
-2 































11 
It 
ii 


l\ 


















11 


ll 


e' X 12' CURVED 
REFLECTOR 








'i 




1 














1 


I 




\ 




















1 








— — 






4 








e'x a' PLANE 
' REFLECTOR 


— ^- 




^ 


^ 




.. — 


j 1 


^ 








1 


■ 1 
1 
I 








1 1 
1 r 
1 1 


1 
1 

1 


\ 




/ 


1 
1 


1 1 








It 
II 
II 


i/ 




\ 




1 


II 
w 

II 










II 
II 


\ 





5 -2.0 -1.5 -1.0 -0.5 0.5 1.0 1.6 2.0 
AZIMUTH IN DEGREES FROM MAJOR LOBE 



Fig. 31 — Estimated rtidiation pattern of 6- X 8-foot plaue and 8- X 12-foot 
curved reflectors. 

The gains of a "periscope" system for spacings between 25 and 100 
feet are not shown in Fig. 30. It suffices to say that the combination of 
the 5-foot paraboloid and the 6- X 8-foot phino reflector with these 
separations will yield a gain approximately 42 db. 

4.5.4 Towers 

Both guyed and self-supporting towers are available for the TJ 
system. The guyed tower is made of galvanized structural steel, triangu- 
lar in cross section, 4 feet on a side, and is availal)lc in heights of 80 to 
300 feet in increments of 20 feet. The structure is guyed in three direc- 
tions with guy directions spaced at 120°. Amounting facilities for three 
passive reflectors of either the 0- X 8-foot or 8- X 1 2-foot type, or com- 
binations of the two, are provided. The center positions of the three re- 
flectors are separated by 120° and, with the flexibility inherent in the 
mounting and in the reflector itself, each reflector may be adjusted in 
azimuth through ±40 degrees from its center position. 

The self-supporting tower is made of galvanized structural steel, square 
in cross section, with a straight top section and below this a uniformly 



THE TJ RADIO RELAY SYSTEM 859 

tapering body. It is available in heights of 40 to 300 feet in increments 
of 20 feet and provides mounting rings at the top of the tower to which 
may be attache<l any cnmliination of up to four 6- X 8-foot reflectors, 
8- X r2-foot reflectors and 10-foot paralinloldal antennas. The an- 
tennas may be attached to the mounting rings in any position irre- 
spective of the tower orientation and, with the adjustability inherent 
in the antennas, two adjacent antennas may be oriented to a mini- 
mum angular separation of 25°. 

Both of these towers have been designed to tilt no more than ±5 de- 
gree and to twist no more than =t| degree under wind loading of 20 
lbs per square foot (approximately 70 mph), and to withstand winds of 
100 mph with the maximum number of antennas attached. Ground wires 
and rods to protect foundations and anchors from lightning are supplied 
with both types of towers. 

4.6 Connecting Circuits 

4.6.1 Type A and Type B Entrance Links 

Wlien L-carrier multiplex signals are connected to a TJ radio system, 
amplifioation is required in both directions of transmission. To protect 
against amplifier failures and facilitate maintenance, protection or stand- 
by circuits can lie provided on either a maiuial or automatic basis. The 
complete ensemble of amplifiers, switches and control circuits is referred 
to as an entrance link. Fig. 32 is a block schematic showing the basic 
features of an L-carricr-T,( radio interconnection. When fully 
equipped, three-wire linos with a common protection circuit will supply 
up to three TJ radio channels. Essentially, the only difference between 
the type A and type B links is the maximum permissible circuit length. 
Each circuit on the type A link uses one 40-db flat-gain amplifier and, 
in the type B link, this is augmented by a second amphfier having a 
gain shape complementing the cable loss. 

4.6.2 ON-2 Mnhiplex 

ON-2 multiplex equipment basically provides 48 channels in the 36- to 
2G8-k(5 band, with a transmitted carrier between each pair of voice 
channels. For radio applications, modulating equipment is available to 
stack a second group of 48 channels on top of the basic group to provide 
a 9(>-channel system. The frequency band of the upper group is 310 to 
548 kc, 

The levels out of the multiplex et^iuipment are adequate to drive the 



860 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1960 



FROM L 
MULTIPLEX - 
EQUIPMENT 



TO L 
MULTIPLEX - 
EQUIPMENT 



LLU 



CONTROL LEADS 



INPUT 
SWITCHES 



r 



64 KC 
PILOT MONITORS 



rrn 



OUTPUT 
SWITCHES 



TO 
OTHER 

TJ 
CIRCUITS 



h-"-L-— H 



OUTPUT 
SWITCHES 



64 KC 

PILOT MONITORS 



J 



INPUT 
SWITCHES 



FROM 
OTHER 

■ TJ 

CIRCUITS 



[Z^ MULTIPLEX 
^-^ OUT 



lie 



TJ 
DIVERSITY 

SWITCH 
UNIT 



tHjrN"'"""'' 



64 KCS PILOT LEVEL, 
14 DB AT ZERO 
TRANSMISSION LEVEL 



CONTROL LEADS 



r&l TRANSMISSION 
'<> LEVEL 





L- MILES (MAX) 


LINK 
TYPE 


754 


I6PSV 


0.27" 
COAX 


0.375" 
COAX 


A 


0,51 


0.78 


1.17 


1.75 


8 


- 


3.25 


5.4 


7.9 



Fig. 32 — L-carrier to TJ radio entrance link. 

TJ equipment directly, so that no special entrance link facilities are 
needed. In Table III the channel levels at the point of connection with 
the TJ equipment are summarized and expressed in terms of the trans- 
mitted carrier level at the point in question. 

4.6.3 Television Terminals 

When a television signal is transmitted over radio, it wUl generally 
be supphed from a customer or television operating center (TOC) at a 



Table III 



Location 


Test Point 


MX IN 


MX OUT 


Terminal 
Dropping point 


-40 dbm 
-34 dbm 


-15 dbm 
-35 dbm 



THE TJ RADIO RELAY SYSTEM 



861 



level of 1 volt peak-to-peak (0 dbv). Likewise, at the receiving end of 
the radio system it must be presented to the customer or TOC at the 
same level. In most instances, the television terminal etiuipment and the 
radio equipment will be physically separated, so that connecting facihties 
must allow for cable loss and cable equalization. Fig. 33 shows typical 
connecting circuits when the TJ system is used as an intermediate link 
between two television operating centers. 

Before being applied to the radio equipment, the low-frequency com- 
ponents in the TV signal are reduced in level relative to the high-fre- 
quency components by a pre-emphasis network. At the receiving end, a 
complementary de-emphasis network restores the high- and low-fre- 
quency components to tlieir original relative magnitudes. The purpose of 
the pre-emphasis network is to reduce the modulation index at low fre- 
quencies to reduce differential phase and gain distortion. 

Alternative connecting circuit arrangements will be used when the 
television signal is transmitted or received over unbalanced cables. 
Functionally, they are the same as the balanced arrangements and differ 
only in the types of networks used for equalization and pre-emphasis. 

4.7 Slaiid-by Power 

A 5-kw gasoline engine alternator and a 10-kw diesel engine alterna- 
tor with automatic controls have been made available for use with the 



TRANS MITTfNC 



400' CABLE 



FROM TV 
OPERATING - 

CENTER 




400' CABLE 



RECEIVING 



TO TV 

OPERATING 

CENTER 



r.:.:x^ 



RESTORER 



CLAMPER 
AMPLIFIER 



(±1 



SINE WAVE IN OaV 

FOR 8MCS PEAK-TO-PEAK 

DEVIATION 



NONDIVERSITY 
CONNECTING PANEL 



Fig. 33 — Video terminal connecting circuit. 



862 THE BCLL SYSTP:M technical journal, JULY 1960 

TJ system. When commercial power fails, these units take approximately 
30 seconds to reach operating speed and voltage. A further 20 seconds is 
required for the radio equipment to resume transmission. 

For those applications where a 50-second interruption in transmission 
cannot be tolerated, other types of reserve power plants have been used, 
which provide essentially no-break operation. This is accomplished by 
the use of a motor-generator-flywheel combhiation coupled to a gasoline 
engine through a magnetic clutch. When commercial power fails, the 
clutch engages and the flywheel supplies sufficient energy to carry the 
load and start the gasoline engine. 

V. EQUIPMENT FEATURES 

5.1 Transmitter-Receiver Bay 

5.1.1 General 

The TJ radio transmitter-receiver bay is illustrated in Fig. 34. It con- 
sists of a 6-foot floor-supported duct-type framework built to accept 
standard 19-inch panels. The backplate support for the rf channelizing 
networks extends above the bay framework, giving an over-all height of 
6 feet 8 inches and a total width of 20^ inches. 

The upper third of the bay houses the transmitter, while the middle 
and lower third contain the receiver and power supply, respectively. 
All units and controls arc accessible from the front of the bay, thus 
permitting back-to-back or back-to-wall floor plan arrangements. To 
simplify maintenance and reduce out-of-service time from equipment 
failures, all units except the receiver modulator are of the plug-in type. 
The removal of any unit automatically shuts down the bay as a protec- 
tion against exposure to hazardous voltages. 

External connections to the bay consist of the ac power conduit, 
the transmitter and receiver waveguide runs and a control lead plug, 
which provides alarm and comparator information for the auxiliary bay. 

5.1.2 Radio Frequency Components 

From an rf equipment standpoint, of special interest are the receiving 
modulator and transmitter afc networks. Instead of using standard 
cross-guide fabrication techniques, the waveguide network configurations 
are milled from sectionalizcd aluminum blocks. The blocks are then as- 
sembled to form a relatively compact, composite structure typified by 
the AFC network illustrated in Fig. 35. 



THE TJ RADIO RELAY SYSTEM 



863 




Fig. 34 — TJ fraiiHmitter-receiver bay. 



864 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1960 




Fig. 35 — Tranamitter afc discriminator and power monitoring network. 



5.1.3 Intermediate Frequency and Baseband Units 

Each IF or baseband unit is assembled in a die-cast aluminum chassis. 
Typical of the if units is the main if amplifier illustrated in Fig. 36. The 
components for each stage are mounted on an individual plate assembly, 
which is attached perpendicularly to the tube base of that stage. The 
individual plates are assembled and wired separately, and then mounted 
on the die-cast aluminum chassis prior to interstage wiring. In addition 
to mounting and accurately positioning components, the plate assemblies 
act as interstage shields. The only adjustments on the if units are on 
the input and output coupling networks, which are adjusted for opti- 
mum return loss. 

The baseband amplifiers use standard assembly techniques, with spe- 
cial precautions being taken to accurately position components. This 



THE TJ RADIO RELAY SYSTEM 



805 




Fig. 36 — Pliotograph ot main if amplifier. 

minimizes variations in parasitic capacities to obtain reproducible high 
frequency gain characteristics. 

All units are equipped with test points for measuring plate and heater 
voltages. Test points are also provided to measure tube biases for cath- 
ode activity tests. 

5.1.4 Power Supply 

An internal view of the TJ power supply is shown in Fig. 37. The 
hinged front cover provides access to all components and an interlock 
switch protects against exposure to hazardous voltages. Components 
mounted in the rear of the unit, can be reached by lowering the back- 
plate, which is hinged near the lower end. 

When the radio bay is e(|uipped with a transmitter only or receiver 
only, the appropriate outputs on the power supply are equipped with 
bleeder resistors to maintain a constant load on the unit. These loads are 
located within the power supply and consist of resistors mounted in 
fuse-clip holders. The power supply also contains all voltage and current 
metering resistors. 

Connections between the power supply and the rest of the radio bay 
are made through plug-in connectors in the rear of the unit. 

5.2 Auxiliary Bay 

The auxiliary order-wire, alarm and control bay is a 7-foot duct- 
type framework built to accept standard 23-inch panels. The over-all 



8G6 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1960 




Fig. 37 — Internal view of TJ radio power supply. 



THE TJ RADIO RELAY SYSTEM 867 

height, width and depth dimensions are, respectively, 7 feet, 20| 
inches and 16j inches. 

The bay, which is illustrated in Fig. 38, contains a maximum of six 
diversity switch units, the D-type alarm equipment and associated power 
supplies. Like the radio bay, it can be maintained from the front of the 

bay. 

Control lead connections from the radio bays and all external alarm 
leads are brought to a terminal strip at the top of the auxiliary bay. 
Connections from the diversity switches are brought to the same panel, 
permitting cross connections to be made as required in particular appli- 
cations. 

The diversity switch units are built in the form of drawers which can 
be pulled forward on slides for maintenance purposes. Power and signal 
connections are made through a flexible cable detail which plugs into 
sockets in the bay cabling. Switch miits can, therefore, be provided or 

added as needed. 

Power rectifiers providing -48 and +130 volts are located below the 
switch drawers. If these voltages are available from the office supplies, 
the rectifiers can be omitted and replaced by an optional power-connect- 
ing panel. 

5.3 Interconnecting Arrangements 

5.3.1 Antenna Feed 

The dual-polarized signal received by a TJ antenna is separated into 
its two components by a polarizer^* network, which is illustrated in Fig. 
39. The network is located directly behind the antenna and is connected 
to the radio eciuipment through two rectangular waveguides. The polar- 
izer network has a cross-polarization discrimination over the TJ band 
in excess of 40 db, although only 20 db of isolation is realized in practice, 
due to coupling between polarizations within the anteima. 

In a diversity system, the working and protection rf channels are 
connected to opposite polarizations. This permits maintenance and sys- 
tem additions to be carried out while service is maintained on the oppo- 
site polarization. If only one transmitter and receiver are connected to 
the antenna, the normal channel-dropping network can be omitted by 
connecting the receiver to one polarization and the transmitter to the 

other. 

The rectangular waveguide connections from the polarizer are con- 
nected to the radio bays through a pressure window. Air line connections 



868 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1960 







r-:". V v;* 







Fig. 38 — TJ radio auxiliary bay. 



THE TJ RADIO RELAY SYSTEM 



809 



. sr* *i;K-,ifr^*^^'ah 




Fig. 39 — Polariziitioii scparjitiou network, 

to the pressure window from a dehydrator keep the entire waveguide 
run filled with dry air at a pressure of five inches of water. 

5.3.2 Radio Bay Interconnections 

A typical arrangement for connecting the radio equipment to the an- 
tenna system is illustrated in Fig. 40 for a fully eciuipped diversity 
system. 

When the system has expanded to four two-way radio channels, an 
isolator is provided between transmitters and receivers, as illustrated 
for the six-channel case m Fig. 40. The purpose of the isolator is to pre- 
vent beat oscillator leakage from a receiver on one polarization going 
back through the polarizer network and being picked up by a receiver 
on the opposite polarization. With fewer than four channels the receivers 
are protected from this type of interference by adequate frequency 
separation. 



870 THE BELL SYSTKM TECHNICAL JOURNAL, JULY lOfiO 




h^-f-i 



DIVERSITY 
PAIRS 



i-i~-i 



ISOLATOR 



CHANNEL SEPARATOR, 
COMBINING NETWORK 




Fig. 40 — Waveguide interconnecting arrangements for a three-channel di- 
versity system. 



VI. SYSTEM MAINTENANCE AND TEST EQUIPMENT 

Wherever possible, test procedures have beeji based on the use of 
commercially available test equipment. The only specialized test sets are : 

(a) an impedance matching test set to enable commercial oscillators 
and voltmeters to be used with TJ circuit impedances; 

(b) an IF test set for measuring intermediate frequencies and power 
output ; 

(c) an RF test set for generating and measuring 11,000-mc microwave 
signals; 

(d) a transmitter disconnect unit, used to remove one transmitter of 
a diversity pair from service without introducing sudden level changes 
in the other which might affect data circuits. 

The IF test set is illustrated in Fig. 41. It contahis an if frequency 
meter and attenuator, together with a calibrated detector for measuring 
IF signal level. 



THE TJ RADIO RELAY SYSTEM 



871 




Fig. 41 — IF test set. 



VII. APPLICATIONS OF TJ RADIO SYSTEMS 



7.1 Path Selection 



7.1.1 General 

A number of factors must be carefully considered in engineering any 
microwave relay system. Before getting down to actual site selection it 
is necessary to consider all information known about propagation con- 
ditions in the general area. Tor instance, experience has shown that the 
inverse bending (A:<1) type of fading is more prevalent in the moist 
coastal regions, and that some additional tower height should be con- 
sidered for improved reliability in these areas {k is the radius of a fic- 
titious sphere relative to that of the earth and, under normal propagal ion 
conditions, is e(iual to 4/3). This type of fading is not frequency-selec- 
tive, and propagation at 4,000, ti,000 and 11,000 mc will be affected 
similarly. 

On the other hand, multipath fading at 1 1 ,000 mc is dissimilar to that 
at 4,000 mc in certain aspects. Multipath fading is due to reflections 
from stratifications set up in the atmosphere that provide two or more 
paths of different electrical length for the radio signal. AVhile the time 
of occurrence and duration of this type of fading at 11,000 mc is ap- 
proximately the same as it is at 4,000 mc, the received signal levels vary 
more rapidly at 11,000 mc. Multipath activity is more prevalent at 
night tlian during the day; it is greater in warm and humid .seasons than 
in cool and dry seasons; it is greater over water or smooth terrain than 



872 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 19G0 



over rough ground and is more severe on longer paths. This type of 
fading is frequency -selective. 

7.1.2 Path Clearances 

Recommended path clearances for the TJ system are shown hi Fig. 42. 
These clearances are for light-route systems in general and should be 
applied over true earth. For over-water paths it is recommended that 
they be increased by D-/24: feet, where D is the path length in miles, in 
order to provide adequate mid-path clearance for protection against in- 
verse bending type of fades. This suggested increase is based on values 
of k of approximately 2/3. 

7.1.3 Received Signal Objective 

Based on a 40-db fading margin and a suitable FM breaking allow- 
ance, the received signal level objective is —35 dbm to —40 dbm, de- 
pending upon the message load. A received signal level of —35 dbm can 
normally be met for path lengths in the order of 25 miles. Practical 




7 a 9 10 20 

PATH LENGTH, D, IN MILES 



30 



40 SO 60 70 BO 100 



Fig. 42 — ■ TJ radio path clearance requirements. 



THE TJ RADIO RELAY SYSTEM 



873 



puth lengths will vary from iihout 10 miles in regions of very severe 
rain to 40 miles or more in the drier climates. 



7.1.4 Overreach 

Since alternate repeaters use the same "nominal" carrier frequencies 
for a given direction of transmission, there will be the possibility of a 
receiver three hops away being subject to hiterference. The carrier fre- 
quencies may dilTer by as mucli as a megacycle or more, and the effect 
of the interference will be to phase -modulate the wanted carrier to pro- 
duce a tone in the baseband at a fre<iuency e(iual to the "nominal" 
frequency difference. Normally, this type of interference is avoided by 
zig-zagging. In laying out microwave routes, path bearings should be 
selected so that a total of 50 db discrknination against overreach inter- 
ference will be provided by the two antennas involved. 

7.1.5 Branching 

The TJ system is designed for a maximum capability of 12 two-way 
radio frequency channels. For a through repeater and no branching radio 
routes, six transmitters will normally be used in each direction to pro- 
vide three working and three protection channels. 

Because no frequency is reused at a given repeater, any branching 
route configuration desired may be used, provided the total number of 
two-way radio channels does not exceed the system capability of 12. 
Fig. 43 shows a typical branching or spur route. 



(H) 11,645 (3B) [ "k^ _ ^ ^L-^ 

(V)11,405CIB) 1 K * S I 



TO TRAN5MITTERS 

RECEIVERS 



TRANSMITTERS 



> — (V)10,995[3A) 



> — (H)10,775(IA) 



-(V)11,565(7B)— C 
(H)11.325(5B) [_ 



> ^ n (H)I0,9f5{7A) 
' N I (V)ll,t55(5A) 



(H) 11,485 Cl'B) -C 

(V)1I, 245(98) C 




TO RECEIVERS 



'°7< 



^<Hu, ''^'-'m; 







Fig. 43 — Typical frequency assignments at TJ branching point. 



874 THK BRLL SYSTEM TECHNICAL JOURNAL, JULY 19C0 

There will be instances where it will become desirable to add a branch- 
ing route to a fully loaded route. To do this, some radio channel fre- 
quencies must be reused at the branching point. In these cases, careful 
consideration must be given to near-end and far-end crosstalk. A require- 
ment of 50 db must be placed on the reduction of any undcsired signal 
arising from these exposures. This precludes the use of periscope an- 
tennas. In order to meet this requirement, paraboloidal antennas will 
have to be used at the branching point on those channels having the 
same fre(iuency. In addition, frequencies should be selected so that the 
channels common to both routes have the largest angular separation pos- 
sible. 

7.2 Typical TJ Installations 

7.2.1 Phoenix-Flagstajf System 

Prior to the installation of the TJ radio system between Phoenix and 
Flagstaff, Arizona, the facilities along this route consisted of an open- 
wire line equipped with various types of carrier. By 195G these facilities 
had been developed to nearly their maximum capacity. New circuit 
requirements for northern Arizona would necessitate either an expansion 
of the old route or construction of a new route. The age and condition 
of the open-wire route made it uneconomical to rebuild this facility. 
Cost studies indicated that construction of a new wire-line facility would 
cost less initially, but that for a cross section greater than 60 two-way 
circuits, radio multiplexed with L-carrier would be more economical. 
Since the estimated re(iuirements for the 20-year engineering period were 
in excess of 400 circuits, it was decided to develop a new route employing 
radio. 

The Phoenix-Flagstaff TJ radio route, shown in Fig. 44, extends north- 
ward from Phoenix 133 miles, and elevations vary from a little over 1000 
feet at Phoenix to more than 9000 feet at Mt. Elden. Short self-support- 
ing towers, 60 to 120 feet high, and 10-foot paraboloidal antennas are 
employed on all paths with the exception of the Mt. Elden-Flagstaff 
path, a distance under four miles. Five-foot paraboloidal antennas were 
adequate for this short path and no tower was required at the Flag 
staff main office. Mingus Mt.-Mt. Elden is the longest path, in excess 
of 47 miles. Adecjuate clearance is provided by the mountainous terrain 
and attenuation due to rainfall is not expected to be serious in the dry 
Arizona climate. 

It is interesting to note that freight-type containers, 2S^ feet long by 



THK TJ RADIO RELAY SYSTEM 



S75 



ELDEN 
{FLAGSTAFF R IS) 



MINGU5 
(PHESCOTT BIN) 




FLAGSTAFF 



fs 



WINSLOW 



BLACK MESA 
(PRESCOTT R1S) 



DESERT HILLS 
(PHOENIX R1N2) 

26 MILES 

PHOENIX 



; 



TOTAL DISTANCE, 
FLAGSTAFF TO PHOENIX = 137 MILES 

PRESENT SYSTEM 

PROPOSED EXTENSION 
AND SIDE-LEGS 

[~~1 REPEATER STATIONS 

O MP -MAINTENANCE POINT 

(55 MP, CP-MAINTENANCE POINT 
AND CONTROL POINT 
(ALARM CENTER) 



Fig. 44 — Route plan of the Phoenix-Flagstaff TJ system. 

8 feet wide, were used for repeater buildings on this route. These build- 
ings were temporarily located in Phoenix, where the radio equipment was 
installed and a large portion of the equipment tests were made. In this 
way, most of the time normally cons\mied in travelling to and from re- 
peater sites for initial equipment line up and tests was saved. The build- 
ings were then transported to the repeater sites as a complete package 
with the exception of the stand-by power sets. Commercially available 
"no-break" power sets were installed on this system at all locations 
except the Phoenix main office, where there was sufficient capacity in 
the existing L-3 motor alternator to power the additional load. 



7.2.2 Kiptopeke-Hampton Sijstem 

This TJ system, shown in Fig. 45, extends northward from Hampton, 
Virginia, 22.5 miles across Chesapeake Bay to Iviptopeke on the Eastern 
Shore of Virginia. Estimates of rain attenuation for this section of the 
countrv indicate that reliable transmission can be achieved over these 



876 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1960 




Fig. 45 — Hampton-Kiptopeke TJ system with proposed additions. 



distances provided the system has a fading margin of 40 db. However, 
in engineering a microwave system to operate on a long over-water path, 
two propagation phenomena besides rain attenuation deserve careful 
consideration. These are (a) fading due to inverse bending, and (b) re- 
flections from the surface of the water. In the former case, additional 
tower height is usually provided to assure reliability. In the latter case, 
some arrangement, such as a high tower at one end and a low tower at 
the other end of the path, is employed to place the reflection point ad- 
vantageously so that cancellation wiU not occur. As can be seen, the 
remedy for inverse bending is not the remedy for reflective paths. 

The Kiptopeke-Hampton TJ system employs periscope antenna sys- 
tems at both ends of the path. The reflective antenna height at Kipto- 
peke is 266 feet, and at the Hampton end it is 260 feet. The received 
signal level is —32 dbm and, with 36 channels of single -sideband sup- 
pressed-carrier multiplex, 48 dba at TLP occur at a receiver input of 
— 75 dbm resulting in a fading margin of 43 db. The antenna heights 
provide grazing clearance when k ^ 1/2. Under normal propagation 



THE TJ RADIO RELAY SYSTEM 877 

conditions, k = 4/3, the mid-path clearance is in the order of 15 fresuel 
zones. Frequency diversity, with 240-mc channel separation, provides 
reliable transmission during periods when reflection is taking place from 
the water. 

VIII. ACKNOWLEDGMENT 

The system described here is the result of the efforts of several depart- 
ments of Bell Telephone Laboratories, including research, systems de- 
velopment, device development and systems engineering. 

REFERENCES 

1 Grieaer, T. J. and Peterson, A. C, A Broadband Transcontinental Radio Relay 

System, Elect. Engg., 70, 1951, p. 810. 

2 Roetken, A, A., Smitii, K. D. and Friis, R. W., The TD-2 Microwave Radio 

Relay System, B.S.T.J., 30, 1951, p. 1041. 
3. Bell Lab. Rec, 24, 1946, p. 175. 

4 Tillotaon, L. C, A Short-Haul Microwave Transmitter, Bell Lab. Kec, 33, 

1955, p. 131. 

5 McDavitt, M. B,, 6000-MegaeycIe-per-Second Radio Relay System for Broad- 

band Long-Haul Service in the Bell System, A.I.E.E., General Meeting, 
October 1957, paper 57-1044. 

6. Cronburg, C. I. L., Jr. and Aruck, M., 96-Channel Multiplex for ON Carrier 

on Radio, A.I.E.E., Seattle Meeting, June 1959. 

7. Hathaway, S. D. and Evans, H. W., Radio Attenuation at 11 kmc and Some 

Implications AITccting Relay System Engineering, B. S.T.J. , 38, 1959^ p. 73. 

8 Pound R v.. Electronic Frequency Stabilization of a Microwave Oscillator, 
Rev. Sci. Inst., 17, 1946, p. 490. 

9. Pomeroy, A. F., to be published. , . ^., r ^,- 

10 Lewis W. D. and Tillotson, L. C, Non-Reflecting Branching Filter for Micro- 
waves, B.S.T.J., 27, 1948, p. 83. 

11. Small, R. H., to be published. 

12. Dahlbom, C. A., A Transistorized Signaling System, Bell Lab. Rec, 37, 1959, 

p. 254. 

13 Jakes W. C, Jr., A Theoretical Study of an Antenna-Reflector Problem, Proc. 

I.R.E., 41, 1953, p. 272. ^ ,,^^ ^ 

14 Ohm E A., A Broadband Microwave Circulator, I.R.E. Trans., MTT-4, 

1956, p. 210.